1. Field of the Invention
This invention relates to a method of evaluating a color filter to be employed in a liquid crystal device, a solid-state image pickup device, etc., and also relates to a color filter evaluated by the method and to a liquid crystal display device which is provided with such a color filter.
2. Description of the Related Art
Recently, because of advantages such as space-saving, lightweight properties, power-saving, etc., due to slimming-down of a device, a liquid crystal display device is now rapidly propagated especially as a display device for a television. In order to make the liquid crystal display device applicable to a television, it is required to further enhance various properties such as a brightness, contrast and omnidirectional visibility, therefore the display device is now frequently constructed such that an optical retardation control layer is employed in combination with a linear deflecting plate.
In recent years in particular, in the case of a vertical alignment mode liquid crystal display which is capable of exhibiting a display of high contrast, there has been employed an optical retardation film exhibiting negative birefringence anisotropy with the optical axis thereof being perpendicular to the substrate thereof (or negative C plate) together with an optical retardation film exhibiting positive birefringence anisotropy with the optical axis thereof being horizontal to the substrate thereof (or positive A plate) (for example, see JP-A 10-153802).
In order to realize such an optical retardation control as described above, there have been generally employed an optical retardation control film that can be obtained by stretching an polycarbonate film or that can be obtained by coating a liquid crystal material exhibiting birefringence anisotropy on the surface of a triacetyl cellulose film, etc.
However, since the retardation of the aforementioned optical retardation film is uniformly retained in-plane and hence the retardation is not optimally set for each of pixels to be actually displayed, it cannot necessarily be said that compensation of the optical retardation is optimally executed by the optical retardation film.
One of the reasons is that since the optical retardation and refractive index of liquid crystal material themselves are dependent on the wavelength of transmitted light, the retardation demanded for the optical retardation film may differ depending on the color of each pixel constituting the color filter (actually, on the wavelength of transmitted light). In view of this, it has been proposed to control the retardation in conformity with the wavelength of transmitted light, thereby making it possible to optimize the compensation of optical retardation (see, for example, JP-A 2005-148118).
Another reason is that when each of color pixels constituting the color filter has in itself a perpendicular (thickness-wise) optical retardation, an optical retardation is caused to generate in transmitted light, so that the viewing angle dependency of a liquid crystal display device becomes larger, thereby deteriorating the display characteristics thereof. In view of this, it is proposed to construct the colored layer constituting the color filter in a manner to contain a polymer having a planar structure group on its side chain or in a manner to contain birefringent-reducing particles having a birefringence index which is opposite in sign (positive or negative), thereby trying to reduce the retardation of the color filter (see, for example, JP-A 2000-136253).
As a matter of fact, however, in spite of these attempts, there is a problem that when dark display in off state to which viewing angle compensation has been applied is observed obliquely, the dark display appears colored into reddish purple due to the leakage light of red color and blue color.
As a result of studies made by the present inventors on the cause of this problem, it has been found out that the perpendicular optical retardation of each of red, green and blue pixels constituting the color filter differs from each other and that, in the case of the color filter to be manufactured by making use of a pigment dispersed color composition, a green pixel is enabled to exhibit a large negative retardation as compared with the retardation of red and blue pixels, thereby raising the aforementioned problem.
Since the retardation of a color filter is relatively small as compared with that of other components to be employed in a liquid crystal display device, the aforementioned problem was not considered seriously up to date. However, since the optical designing is now generally performed centering around the green color, if the retardation of green pixel differs greatly from that of red and blue pixels, leakage light is caused to generate, thus raising problems with respect to the view angle visibility of the display device.
Generally, the perpendicular optical retardation of each of color pixels of red, green and blue colors constituting the color filter can be measured by making use of an ellipsometer or a phase shift-measuring apparatus. It has been considered difficult, however, in the employment of the conventional technique, to precisely measure the perpendicular optical retardation by making use of a specific wavelength due to the facts that the thickness of the color pixel formed on a substrate falls in most cases is within the range of 1 μm to 3 μm and that the refractive index of the color pixel falls in most cases within the range of 1.55 to 1.8, thereby enabling the influence of interference due to a difference in refractive index between the color pixel and the air layer or the substrate to be included in the phase shift Δ obtained, i.e. the value measured.
With respect to the view angle visibility especially from an oblique angle, since it may be influenced also by the balance of transmissivity of each of red, green and blue pixels constituting the color filter, it has been considered difficult to discuss the visibility simply on the standpoint of how to control the perpendicular optical retardation.